A common practice in forming fibers of mineral material, such as glass fibers, is to pass the material in the molten state through the orifices of the peripheral wall of a centrifuge or spinner to create primary streams of molten glass. Thereafter, the primary streams are further attenuated into glass fibers of smaller diameter by the action of a flow of gases ddischarged downwardly in an annular jet positioned circumferentially of the spinner. Some fiber forming processes, such as the Supertel process, use an annular combustion chamber burner positioned around the spinner to produce a "flame" or gasses of combustion resulting in a high-velocity, high-temperature gaseous jet passing across the face of the spinner.
The pirmary streams of molten glass emanate from numerous rows of orifices in the spinner peripheral wall. Typically, in the production of glass fibers for insulation products or structural products, the orifices are closely packed on the spinner peripheral wall, numbering 10,000 or more orifices per spinner. The close spacing of the orifices in the spinner peripheral wall results in bumping and interference among the primary streams and glass fibers during the formation of the glass fibers. There is little or no effect on the ultimate insulation or structural products, however, because these fibers are thick enough to withstand the contact without deterioration. These insulation products typically have fiber diameters ranging from 4 to 8 microns for insulation products, while structural products such as molding media and pipe insulation material have fiber diameters ranging from 5 to 15 microns.
When the rotary process is used to produce smaller diameter staple or chopped fibers for such critical uses as the production of wet process mats, it has been found that fiber interference during fiber forming greatly impairs the strength of the individual fibers. This results in ultimate products (e.g., reinforcing mats made from discrete length glass fibers) having unacceptably low strength. In order to increase the strength of the rotary chopped fibers to be used as input for wet process mats, it is necessary to minimize or eliminate fiber interference during fiber forming. Also, steps taken to minimize fiber interference must be done with a view toward economics, which require certain throughput levels per spinner for economic efficiency. Although the possibility exists of making high-strength rotary fine fibers using a single row of holes, this could not be considered an economical process.
One method proposed in the past for eliminating fiber interference in the rotary process is that disclosed by Charpentier et al. and U.S. Pat. No. 3,304,164. The method disclosed is to provide the spinner with orifices of greater diameter in the upper rows and orifices of lesser diameter in the lower rows. The primary streams issuing from the upper part of the spinner are thicker and have a greater mass flow rate and are therefore deflected less by the jet from the annular combustion chamber burner. Charpentier et al. teach that the result of the gradient of orifice diameters is less fiber interference, resulting in greater tensile strengths for the glass fibers.
The teaching of Charpentier et al. breaks down when applied to a process for making fine fibers having diameters smaller than about 3 microns. These fine fibers are more likely to be used for making glass fiber mats which require tensile strengths. Throughput per hole is so low that under normal production conditions any minor upset in the production process translates to significant deterioration of fiber tensile strength. This happens because of maldistribution of throughput across the face of the spinner, giving rise to interference of lower fibers with the upper ones. Accordingly, due to the difficulty of maintaining a stable production process and the extreme sensitivity of fine fiber tensile strength to defects caused by interference, production of fibers in the desirable diameter range 1/2-3 microns) is very difficult, especially at commercially acceptable throughputs.